Extrachromosomal elements, plasmids, in bacteria are now well-recognized and common entities. Yet, the control of their maintenance in the host cell is largely unknown. Our studies and those of others suggest that plasmids are not freely diffusible in the cell, but are sequestered in particular sites which appear to be specific for a plasmid or group of plasmids. We have, for instance, demonstrated that some, but not all plasmids will segregate into E. coli minicells which form at the cell poles. F plasmids do not normally segregate, but mutations in regions close to the origin of replication allow the plasmid to now segregate into minicells. The use of different segregating and non-segregating mini F plasmids has introduced a system in which to study the control of this phenomenon which appears to illustrate a form of plasmid compartmentalization in the cell. Our studies will identify genes which control mini F plasmid segregation and identify those genes which appear to inhibit F and R222 segregation into minicells. Studies will attempt to characterize the biologic process and to identify the protein or RNA effectors involved in this process. These studies deal with a basic property of plasmids which has not previously been addressed. Findings should have both specific and general implications in biology.